Self-Structuration of Three-Wave Dissipative Solitons in CW-Pumped Optical Cavities

Abstract

Generation of ultra-short optical pulses in cw-pumped ring cavities are mostly associated to mode locking in active media, as doped fibers or solid-state (e.g. Ti-Sa) lasers. The cavity contains not only a gain element (atoms or ions) but also a nonlinear element of the host medium, such as self-phase modulation (SPM) or intensity dependent absorption. Spontaneous generation of a pulse train in cw-pumped optical fiber cavities without gain elements can been also obtained through modulation instability caused by the combined action of SPM and group-velocity dispersion (GVD) on the CW optical beam [1]. Our aim here is to present another mechanism for pulse generation in a ring cavity due to the three-wave counterstreaming interaction. In this case, nanosecond pulses are spontaneously generated in a cw-pump Brillouin-fiber-ring laser [2]. We show that the same three-wave counterstreaming interaction responsible of symbiotic solitary wave morphogenesis in the Brillouin-fiber-ring laser [3] may act for picosecond pulse generation in a quadratic optical cavity (optical parametric oscillator) [4]. The resonant condition is automatically satisfied in stimulated Brillouin backscattering (SBS) when the fiber-ring laser contains a large number of longitudinal modes beneath the gain curve, since the cw-pump selects among them the resonant acoustic wave (of wavelength near the half of the pump- or Stokes- wavelength). However, in order to achieve quasi-phase matching between the three optical waves in the x ⁽²⁾ medium a grating of sub-μm period is required. Recent experiments of backward second-harmonic generation in periodically-poled LiNbO₃ [5, 6] avoids this technical difficulty by using higher-order gratings.